This invention relates to a method and apparatus for forming features in cans. In particular, it relates to a method and apparatus for mechanically reshaping and/or forming textured features in the side wall of metal can bodies.
It is known for example from EP-0492860 that features such as flutes can be formed in the side wall of can bodies by rolling the can, supported on a hard profiled mandrel, along a flexible rail of polyurethane. In EP-0492860, the profile of the mandrel comprises a whole number of flutes which is less than the number of flutes on the finished can body.
EP-0731740 describes another apparatus for forming grooves such as flutes in a can side wall. The apparatus of this application uses a rail having hard profiled features, the can body being carried by a mandrel of softer resilient material such as polyurethane.
EP-0492860 also describes the use of a rail and mandrel, both of which are made from hard material. The rail is fixed in position and very fine clearance and accurate matching of forming depth between the mandrel and rail must be maintained for the flutes to be formed. It is not feasible to maintain these in practice due to the increase in temperature and machine and tool expansion which occur during normal running conditions. Typically a rise of up to 40.degree. C. is found when operating at 500 cans/minute and a temperature rise of 50.degree. C. has been found when operating a beader at 1500 cans/minute. Since compensation for this temperature rise is not possible, damage to the machine can occur.
A roll forming apparatus such as that described in EP-0492860 uses a rotating turret to carry a number of heads comprising profiled mandrels, each of which is rotatably mounted on the turret on shafts. As the turret rotates, the can bodies located on the profiled mandrels are engaged between a profiled mandrel and a profiled rail. The shafts of the mandrels are driven so that cans mounted on the mandrels are rolled along the rail. The radial position of the mandrels on the turret is set prior to operation. However, if there is mis-setting of the heads, this will lead to variation in the depth of profiles formed on the cans which may be unacceptable to the customer.
It should be noted that temperature rise leads primarily to turret growth and subsequent change in profile depth. This in turn will result in a change in can performance. If the heads have been incorrectly set, then the problem is exacerbated still further.
Another profile commonly provided for food cans is beading. Beading typically comprises one or more clusters of circumferential beads which improve can panel performance (i.e. radial strength when subjected to external pressure) particularly during thermal processing. Beads are generally formed by rolling the can body between a rotating mandrel and a fixed rail, or a pair of rotating mandrels. Both tools are independently mounted and located on separate assemblies. However, as the temperature of the machine and tooling increases during normal operation, the depth of the beads varies and cans with unacceptable bead depths made during the warm up period may be rejected. Conventional beaders have been found to exhibit up to 0.1 mm (0.004") depth growth when hot. One beader, operating at 1500 cans/minute was found to exhibit up to 0.18 mm (0.007") depth growth.
Variation in depth on beading machines has become more of an issue as the industry is continually striving to produces thinner lightweight cans. Previously, body thickness was high enough to absorb changes in bead/profile depth resulting from poor machine settings and temperature variation. This is no longer the case.
None of the prior art documents addresses or even recognises the problem of control of the depth of the profiled features formed in the can body. In particular, the problem of depth variability which will arise due to expansion of the machine and tooling in normal running, poorly set heads, or variability in thicknesses in the can body, has not been previously been addressed. This invention seeks to provide a solution to that problem.